Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, waste from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of chemicals and fuels as are most abundant and lower in cost. Lignocellulosic biomasses are mainly comprised of cellulose, hemicellulose and lignin. Other non-structural components (phenols, tannins, fats, sterols, sugars, starches, proteins and ashes) of the plant tissue generally accounts for 10% or less of the dry weight of biomass.
Different types of biomass, such as woody plants, herbaceous plants, grasses, aquatic plants, agricultural crops and residues, municipal solid waste and manures, contain varying amounts of cellulose, hemicellulose, lignin, and extractives. Cellulose, the most abundant polysaccharide in all plants and consists mainly of glucose, accounting about 50% of the plant weight. The cellulose chain which form fibrils consists of about 10,000 glucose units. The cellulosic material has a crystal domain separated from the less-ordered, amorphous domain, which allows chemical and biochemical attack, whereas; hemicellulose is a short (100-200 sugar units), highly-branched heteropolymer consisting of predominantly xylose as well as glucose, mannose, galactose, arabinose and other uronic acids. C5 and C6 sugars are linked by 1,3-, 1,6- or 1,4-glucosidic linkages. Often these sugars are acetylated at primary hydroxyl groups. Cellulosic fibrils are embedded in an amorphous matrix network of hemicellulose and lignin, and they serve as glues between the plant cells, providing resistance to biodegradation.
Lignocellulosic biomass (LCB) is the most abundant economically available materials across the world and has been considered as a potent source for ethanol production owing to its easy availability and richness in sugars. These sugars on fermentation produce ethanol. LCB comprise of cellulose, hemicelluloses and lignin besides extractives and ash. Cellulose is a polysaccharide consisting of linear chains of P-(1-4)-D-glucopyranose units. Hemicellulose is a heteropolymer composed of xylan, arabinoxylans, xyloglucan, glucuronoxylan and glucomannan, while lignin is a complex biopolymer of mono-lignols deposited in the cell walls of LCB, which basically provides strength to LCB. LCB need to be pretreated to destruct the biomass cell wall matrix to make it amenable to enzymatic hydrolysis. Subsequently enzymatic hydrolysis using cellulases leads to the production of sugars, which on fermentation results in the production of ethanol.
Pretreatment is the most energy intensive and expensive step for the production of ethanol. Various pretreatment technologies have been explored such as mechanical, steam explosion, AFEX, acid hydrolysis, organosolv, alkaline hydrolysis, lime pretreatment, hot water, and ammonia pretreatment.
Among all, alkali pretreatment is receiving increased interest. The use of an alkali causes the degradation of ester and glycosidic side chains resulting in structural alteration of lignin, cellulose swelling, partial decrystallization of cellulose, and partial solvation of hemicellulose. This facilitates enzyme accessibility and improves cellulose conversion.
For alkaline pretreatment, sodium hydroxide has been extensively studied for many years. It has been shown to disrupt the lignin structure of the biomass, increasing the accessibility of enzymes to cellulose and hemicellulose. Another alkali that has been used for pretreatment of biomass is lime. The limitations associated with alkaline pretreatment are low solid loading, slimy nature of biomass, extensive washing step before enzymatic saccharification.
Lately, green solvents like ionic liquids (ILs) have also come up for dissolving cellulose. But, ILs are expensive and need to be synthesized at lower cost and on a larger scale. Another well used solvent N-methyl morpholine N-oxide (NMMO), also known as the Lyocell solvent is used commercially to produce Tencel fibers. NMMO retains all the advantages of ILs and capable of being recovered up to >99% of the solvent. But, the high temperature required and the cost of NMMO recovery calls for further research to evaluate and improve the economics of its usage for pretreatment of biomass and to integrate it with enzymatic hydrolysis.
Cellulose dissolving mixtures received interest for wood pulping and modification of cellulose to form membranes etc. Jhau and Zhang (2000) reported NaOH-urea solvent system to form cellulose membranes with high tensile strength and storage stability (Polymer Journal, 2000 (32) 866-870).
Amine solvents such as alkanolamines, alkylene diamines and polyalkylene polyamides with pronounced basic properties are capable of delignifying wood or other lignocellulosic raw materials to produce pulp. The associated drawback regarding the use of these compounds involves relatively long reaction times and/or high temperatures and pressures to achieve efficient delignification. However, this limitation could be overcome by the use of compounds like quinonoids or hydroquinonoids, which markedly increased the rate of lignin removal during pulping.
Ammonia based processes like ammonia fiber explosion (AFEX) is another process requiring moderate temperatures. Provision to recover ammonia offers the economic viability to the technique. However, limitations can also be seen in the form of costs during recycle of ammonia and treatment of chemicals that are being used. Also, lignin-carbohydrate complexes are cleaved, and the lignin is deposited on the surfaces of the biomass possibly causing blockage of cellulases to cellulose.
Ammonia and amines penetrate into cellulose crystals, forming crystalline complexes (Clark and Parker 1937; Davis et al. 1943; Klenkova 1967). Hydrazine (N2H4), the simplest diamine with ammonia-like odor, is known to dissolve cellulose at high temperature and pressure (Kolpak et al. 1977), but it also forms stable complexes with cellulose I and II under ambient temperature and pressure (Lee and Blackwell 1981; Lee et al. 1983). This interaction has been utilized in many industrial processes to enhance accessibility and chemical (Yanai and Shimizu 2006) and enzymatic (Igarashi et al. 2007) susceptibility of cellulose. Hydrazine is recently highlighted as the fuel for a new-type fuel cell, which may contribute to reducing carbon dioxide emission (Asazawa et al. 2007). The major drawbacks of hydrazine are its toxicity and volatility.
In prior art US 20090298149 A1, sulphite/bisulphite has been used for pretreating wood chips for ethanol production. Sulfonation of lignin increases its hydrophilicity, which will promote the enzymatic hydrolysis process. Additionally, it was also mentioned in the prior art U.S. Pat. No. 5,049,661 A, that the addition of some amount of other salts like sulphites, metal chlorides in dilute acid pretreatment improved the enzymatic hydrolysis by delignifying the biomass, which reduce the cellulase amount significantly and in turn produces low amount of inhibitors.
Thus, there is always a need for the efficient pretreatment step which results in production of low amounts of inhibitors in the pretreatment hydrolysate and retains the maximum amount of cellulose in the biomass.
As mentioned in prior art WO 2015063549, in order to hydrolyze the biomass polysaccharides into fermentable sugars, for example by depolymerization, pretreatment processes such as steam explosion, mild acid treatment, strong acid treatment, ammonia treatment, alkali treatment, etc. are employed. Pretreatment is primarily used to make the polysaccharides of lignocellulosic biomass more readily accessible to cellulolytic enzymes. The ideal pretreatment process should be environment-friendly and economically feasible. The pretreatment method will be selected considering the process dependency and cost, as well as process yield and production parameters. Since the major cost of the overall conversion process is due to the biomass feed pretreatment and enzymes, it is necessary to minimize the use of enzymes and obtain the maximum conversion of the carbohydrates to ethanol. For these reasons, a considerable amount of research work has been done for developing means to pretreat the lignocellulosic biomass in such ways that it becomes more accessible to cellulolyic enzymes.
U.S. Pat. No. 4,178,861 A discloses amine based liquor containing a quinone or hydroquinone compound for delignification of lignocellulosic material for manufacture of paper or paperboard. Of the ethanolamines, mono ethanolamine was the preferred compound. Preferred alkylene diamines are those diamino lower alkanes such as ethylene diamines and propylene diamines. Preferred polyalkylene polyamines are derivatives of the lower alkylene diamines such as diethylenetriamine and triethylenetetramine.
U.S. Pat. No. 4,826,567 A discloses a process for the delignification of cellulosic substances wherein hydrazine was added in the third step of treatment. The previous two steps comprised of treating the biomass with acid and hydrogen peroxide. The invention related to a process for the delignification of lignocellulosic substances for preparation of pulps intended for paper manufacture.
N-methylmorpholine-N-oxide (NMMO)/H2O system developed by Chanzy et al. (Chanzy, H., et al., J Polym Sci: Polym Lett Ed (1979) 17:219-226) has been industrialized for the solvent spinning of cellulose. The product spun by this process is sold under the registered trademarks TENCEL® and COURTAULDS LYOCELL® by Courtaulds Fibres (Holdings) Limited, London, England, United Kingdom. The advantage of this solvent is its ability to attain exceedingly high concentrations of cellulose (e.g. 35% w/w in DP600) and anisotropic solutions, first reported on non-derivatized cellulose. See Chanzy, H. and Peguy, A., J Polym Sci: Polym Phys Ed (1980) 18:1137-1144. However, the NM1\40/H2O system has significant disadvantages associated with its use, e.g. high temperature required for dissolution; the degradation of cellulose; side-reactions of the solvent itself without an antioxidant (Potthast, A., et al., Holzforschung (2000) 54:101-103); and its high cost.
US2003136304 (A1) discloses a solvent system other than NM1\40-/H2O comprising an amine-based solvent for cellulose. Hydrazine (NH2NH2)/salt system was found to be an excellent solvent for cellulose. Even at room temperature the combinations of hydrazine and lithium, sodium, and potassium thiocyanate had high dissolution power for cellulose up to 18% w/w maximum unrelated to the polymorph, while the combination with ammonium thiocyanate exhibited a solubility difference among cellulose I, II, and III.
Additionally, in the prior art U.S. Pat. No. 4,178,861 A, it was noticed that the addition of some amount of other salts like quionones, sulphites, metal thiocyanades improved the delignification of lignocellulosic material or improved the cellulose solubilization, with an implication in improved enzymatic hydrolysis. Therefore, search was made to look in the prior art which uses hydrazine along with other salts like sulphites, metal chlorides. The prior art related to this is given below:
In prior art US 20090298149 A1, sulphite/bisulphite has been used for pretreating wood chips for ethanol production. Sulfonation of lignin increases its hydrophilicity, which will promote the enzymatic hydrolysis process. The lignosulfonate in turn has been used as oil field chemicals, pesticide emulsifier, dyeing and finishing auxiliaries for textile, which can be obtained from the concentrated sulfite pretreated solution. In industrial practice for more than a century, sulfite pulping has been and can be operated over a wide range of pH and temperature. The active reagents in sulphite pretreatment liquor are also depended on the pH of the pretreatment temperature. In the prior art, sulfonation has found to be enhanced enhanced because of the acid or alkaline catalysis.
J. Y. Zhu et al. (“Sulfite Pretreatment (SPORL) for robust enzymatic saccharification of spruce and red pine”; Bioresource Technology 100 (2009) 2411-2418) reports sulfite pretreatment to overcome recalcitrance of lignocellulose for the efficient bioconversion of softwoods.
US 20090298149 A1 describes a method using sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL). More specifically, it relates to a sulfite-based chemical process for pretreating biomass in solutions to reduce access barriers of enzymes to the lignocellulose, resulting in the efficient conversion through enzymatic saccharification.
US 2009/0298149 A1 discloses a method using sulphite-based pretreatment to overcome recalcitrance of lignocelluloses (SPORL). More, specifically, it relates to a sulphite-based chemical process for pretreating biomass in solutions to reduce access barriers of enzymes to the lignocelluloses, resulting in the efficient conversion through enzymatic saccharification.
There is also need in the art to produce ethanol from ligno-cellulosic biomass (LCB) involving the steps of pretreatment, enzymatic hydrolysis followed by fermentation. Particularly, there is also need in the art for a process of pretreatment by which has beneficial effects in the subsequent enzymatic hydrolysis as very low amounts of inhibitors are generated in the pretreatment and high amounts of sugars are released during enzymatic hydrolysis than the methods disclosed in the prior art.
One of the major challenges of biomass pretreatment is to maximize the extraction or chemical neutralization of the lignin, while minimizing the loss of carbohydrate (cellulose plus hemicellulose) via low-cost, efficient processes.
One of the major challenges of biomass pretreatment is to maximize the extraction or chemical neutralization (with respect to non-productive binding of cellulolytic enzymes) of the lignin, while minimizing the loss of carbohydrate (cellulose plus hemicellulose) via low-cost, efficient processes. The higher the selectivity, the higher the overall yield of monomeric sugars following combined pretreatment and enzymatic saccharification.